1. Field of the Invention
The present invention relates to a charged particle optical system comprising an electrostatic deflector for the deflection of at least one beamlet of charged particles, which deflector comprises a first and a second electrode between which the beamlet passes, which beamlet is deflected upon setting a potential difference between the electrodes.
The invention further relates to the use of such charged particle optical system
2. Description of the Related Art
One such charged particle system is known from U.S. Pat. No. 6,897,458. This system is a maskless lithography system. According to this lithography system, a beam of charged particles, such as electrons, is split in an aperture plate into a plurality of beamlets. The beamlets are subsequently focused to a desired diameter and pass a beamlet blanker array comprising blanking electrostatic deflectors. On application of a voltage to the blanking deflector, a beamlet is deflected so as to terminate at a beamlet stop array located behind the beamlet blanker array. Without deflection, the beamlet reaches a set of lenses to focus the beamlet on the target surface. Scanning deflection means move the beamlets together in one direction over the target surface.
Electrostatic deflectors may be used for the blanking deflectors and the scanning deflectors in such a maskless lithography system and in other high-speed deflection applications. Typical examples are oscilloscope tubes, electron beam lithography systems and inspection systems, and streak cameras. A common type of electrostatic deflector is a planar deflector, which comprises two parallel plates with opposite voltages +V and −V. An electric field is therewith generated in the (x−) direction normal to the plates. Such planar deflector deflects a beam in one direction only. The disadvantage of planar deflectors is that x and y deflections must be applied sequentially at different distances (i.e. different z-positions) to a target, for instance a wafer of semiconductor material.
Another type of deflector is a multi-pole deflector, the most common instance thereof is an octopole deflector consisting of curved plates with cylindrical or conical segments. By applying a suitable combination of electrode potentials to the plates, deflections in two orthogonal directions (x and y) can be applied simultaneously. A disadvantage of this deflector type is its complex construction.
U.S. Pat. No. 6,897,458 specifies a specific electrostatic deflector of the planar type for use as scanning deflection means. This deflector comprises electrodes arranged to deflect an assembly of electron beamlets in a single direction. The electrodes may be deposited in the form of strips on a suitable plate. Alternatively, the strip-shaped electrodes may be deposited on the set of projection lenses, at the side facing the target surface, or alternatively on a separate plate between the set of lenses and the target surface.
FIG. 10 shows a diagrammatical cross-sectional view of a portion of this electrostatic deflector 11. The deflector 11 comprises a first strip 131, a second strip 132 and a third strip 133, which are present on a substrate 150. Passing windows 140, e.g. through-holes, extend through the substrate 150 between the strips 131, 132, 133. The system is designed in such a manner that beamlets of charged particles, i.e. electrons, pass through the passing windows 140. The first and third strip 131, 133 are part of the first electrode, while the second strip 132 forms part of the second electrode. Therefore, the second strip 132 has an opposite polarity to that of the first and the second strip 131, 133. In this example, the second strip 132 is the negative pole. On application of a potential difference between the first and the second electrode, an electric field is generated towards the second strip 132. In view of the consecutive row of electrode strips 131, 132, 133 of opposite polarity, the electric field generated between the first and the second strip 131, 132 has a direction opposite to the electric field generated between the second and the third strip 132, 133. As a result, the beamlets 7 are deflected by the electric field in opposing directions as shown in FIG. 10.
This deflection is disadvantageous because a surface area covered by a grid of the beamlets 7 is larger when the beamlets 7 are deflected than when not deflected. That difference in surface area causes problems for writing a pattern on a target surface that is much larger than the surface area. Then, the patterns of neighboring surface areas need to fit together without any undesired overlaps or gaps in between.
Another type of electrostatic deflector is known from EP1993118. This type is a blanker deflector using an array of electrodes protruding from a substrate. The array is designed to enable deflection in two directions simultaneously and to allow individual addressing of individual electrodes in the array. The latter feature results from the requirement that each beamlet in a blanker deflector is to be deflected separately. Holes are present in the substrate between the electrodes—one active, one ground or opposite polarity—to allow any beamlet to pass. The electrodes have a wall-shaped form and may be formed on two substrates stacked together in such a manner that the electrodes at least partially face each other. The height of these electrodes is in the order of 35-50 μm, the mutual distance may be less than 10 μm. In the event that the electrodes are present on the same substrate, the height may be less than 10 μm and their mutual distance in the order of 0.5-2 times their height. The substrate may be thinned below the membrane with the protruding electrodes.
However, this type of deflector has the limitation that it provides insufficient uniformity if intended for application as a scanning deflector. The presence of one of the electrodes on a second substrate leads to the generation of stray fields and hardly controllable effects. This is not problematic for its use as a blanker deflector; if a potential difference is applied between the electrodes in the blanker deflector, a beamlet will be deflected to terminate at a beam stop. A slightly larger or smaller deflection does not matter as long as it terminates anywhere at the beam stop. But when applied as scanning deflector, such variation would immediately result in a decrease of the resolution of the provided pattern. Additionally, the stray fields may lead to a reduction of homogeneity of the beamlet. This may result in insufficient resist development and/or wrong beam positioning, and therewith non-adequate (i.e. failed) pattern generation.
In short, the prior art has shortcomings that are to be overcome by the invention.